In recent years, from a viewpoint of environmental protection, an aliphatic polyester resin using a plant as a raw material has been paid an attention and, particularly, as a carbon neutral material derived from a plant, a polylactic acid resin has been paid an attention. The polylactic acid resin has a high melting point of about 170° C., and can be subjected to melt molding process and, further, since lactic acid which is a monomer is able to be produced by a fermentation method utilizing a microorganism at low cost, the polylactic acid resin is expected as a biomass plastic which can substitute a commodity plastic derived from a petroleum raw material, and is being gradually used. As a main method for producing the polylactic acid resin, there are a ring opening polymerization method of performing ring opening and polymerization of a lactide which is a dimer of lactic acid, and a direct polycondensation method of performing dehydration-polycondensation using lactic acid. Since the direct polycondensation method can directly use lactic acid as a polymerization raw material without via a step of synthesizing a lactide, it is said that the method can produce a polylactic acid resin at low cost, as compared with the ring opening polymerization method. JP-A No. 8-183840, JP-A No. 2000-297145, JP-A No. 2000-297143, JP-A No. 2000-302852, WO 07/145,195 and JP-A No. 2008-260893 disclose a method for producing a polylactic acid resin using, as a catalyst, a volatile organic sulfonic acid compound such as methanesulfonic acid, p-toluenesulfonic acid and trifluoromethanesulfonic acid, but when these catalysts are used, since the catalysts are vaporized during polymerization, they do not effectively work as a catalyst, a polymerization time is long, and there was a problem in productivity. In addition, WO 07/145,195 discloses an organic sulfonic acid metal salt, but since the form is a metal salt, it does not effectively work as sulfonic acid, and since it becomes to contain a large amount of metal, there was a problem of productivity, and thermal stability of the resulting polymer.
It would therefore be helpful to provide a method for effectively producing an aliphatic polyester resin having a high molecular weight and, in the preferred state, a high melting point and excellent thermal stability and hue.